The goals of this project are to characterize lymphocyte Fc receptors genetically and functionally. Recent findings indicate that Fc receptors of B cells and T cells but not UL cells nor macrophages are associated with Ia antigens. In addition, a subpopulation of UL cells, the effector cells in antibody-dependent cell-mediated cytotoxicity was shown not to bear Ia antigens. In the case of the B cell, the Fc receptors and Ia antigens appear to be distinct molecules, and the association appears to be ligand induced. There is also a single polymorphic non-H-2 locus which determined at least 4 distinct antigens associated with the Fc receptor. In one haplotype, this locus has been shown to be linked to the M locus. Studies with pharmacologic agents suggest that B lymphocyte Fc receptors are linked to the lymphocyte cytoskeleton. These findings indicate that Fc receptors may play a key role in immunoregulation.